<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>三大特性</title>
  <style>
    #d1{
      color: red;
    }
    /*!important作用是提高外边距*/
    div{
      color: green !important;
    }
  </style>
</head>
<body>
<div id="d1">
  <p>这是个标签</p>
  <span>这是div里面的span</span>
  <a href="">超链接</a>
</div>
<span>这是div外面的span</span>

</body>
</html>